1. Field of the Invention
The present invention relates to a magnetic recording medium comprising a nonmagnetic support and a magnetic recording layer provided on the support.
2. Description of Prior Art
A magnetic recording medium is widely used as an audio tape, a video tape, a floppy disc, etc. The magnetic recording medium basically comprises a nonmagnetic support and a magnetic recording layer provided on the support, and the magnetic recording layer comprises a binder and a ferromagnetic powder dispersed therein.
A magnetic recording medium is required to be satisfactory in various properties such as electromagnetic conversion characteristics, running endurance and running property. Recently, an 8 mm type video tape recorder or the like has been broadly employed, and hence it is particularly required for a video tape to have high electromagnetic conversion characteristics such as high reproducibility of an original image.
For improving the electromagnetic conversion characteristics of the magnetic recording medium, a ferromagnetic metal powder having high electromagnetic properties (e.g., coercive force) has been utilized in recent days. The ferromagnetic metal powder comprises a ferromagnetic metal such as cobalt and nickel or their alloys, and employment of such ferromagnetic metal powder makes it possible to prepare a magnetic recording medium having high electromagnetic conversion characteristics.
It is generally thought that the electromagnetic conversion characteristics of the recording medium can be enhanced by using more minimized ferromagnetic powder.
However, the above-mentioned ferromagnetic metal powder has a lower hardness than a .gamma.-iron oxide type ferromagnetic powder or a modified .gamma.-iron oxide type ferromagnetic powder containing cobalt, etc., and therefore a magnetic recording layer of a magnetic recording medium prepared in a conventional manner tends to lower in its mechanical or physical strength as compared with a magnetic recording layer prepared by using the .gamma.-iron oxide type ferromagnetic powder or the modified .gamma.-iron oxide type ferromagnetic powder. Hence, the magnetic recording layer of the recording medium using the ferromagnetic metal powder easily suffers from damages caused by contact with a magnetic head, etc. in the course of running procedure. Thus damaged magnetic recording layer is poor in electromagnetic conversion characteristics, and an expected favorable effect cannot be obtained even by using such ferromagnetic metal powder.
For solving the above-mentioned problem occurring in the case of using the ferromagnetic metal powder, an abrasive is generally incorporated into the magnetic recording layer in a large amount so as to form an area of high hardness in the recording layer, whereby the recording layer is prevented from damages caused by contact between the recording layer and a member of running system such as a magnetic head. That is, as the content of the abrasive increases, the area of high hardness naturally increases in the magnetic recording layer, and thereby the recording layer is hardly damaged.
However, in the case of using a large amount of the abrasive for obtaining favorable mechanical strength in the magnetic recording medium using the ferromagnetic metal powder, the amount of the ferromagnetic metal powder should be reduced. As a results, there arises a problem that the obtained magnetic recording medium hardly shows satisfactory electromagnetic conversion characteristics even if the ferromagnetic metal powder is employed.
The improvement of the electromagnetic conversion characteristics mainly depends on an improvement of the ferromagnetic metal powder (nature, amount, etc.), and the abrasive does not directly contribute to the improvement. Therefore, as for a magnetic recording medium requiring particularly high electromagnetic conversion characteristics such as an 8 mm type video tape, decrease of the amount of the ferromagnetic metal powder accompanied by incorporation of a large amount of the abrasive gives a serious problem.
A method of using a small amount of aluminum in the preparation of a ferromagnetic metal powder for preventing sintering of particles of the ferromagnetic metal powder caused by a reduction reaction by heating is also known as described in Japanese Patent Publication No. 59(1984)-19163, but a function of the ferromagnetic metal powder in the magnetic recording layer obtained by this method has not been clarified so far.